Nowadays in microwave communication lines the two most common types of modulation used are frequency modulation (FM) and quadrature amplitude modulation (QAM), and in multichannel lines it is not unusual for signals of both kinds of modulation, which are very different, to be simultaneously transmitted. Therefore, it is increasingly necessary, both to reduce cost and to make efficient use of available bandwidth, that a standby channel provided for such lines be one that can be commonly used by signals of both types of modulation. However, for signals of different types of modulation to be able to commonly use the same standby transmission path, it must be transparent to both types of signals.
Accordingly, an object of this invention is to provide a new and improved relay station employing a common standby channel switching system that eliminates the above disadvantages. A more particular object is to provide a relay station that can generate a control signal for the switching circuits 43 and 46 within the relay station itself without receiving switching information from a remote terminal station.
In keeping with one aspect of this invention, a microwave relay station comprises first and second channels for respectively transmitting two different types of signals, such as QAM and FM modulated signals. These have a common standby transmission channel that is selectively capable of transmitting the two different types of signals. The type of signal currently carried by the standby channel is identified by a detector circuit, such as one that can automatically distinguish differences in the frequency spectrums of the different types of signals. A selecting circuit responds to the output of the detector circuit by selecting between first and second transmitting means in the common channel, thereby dedicating it to the identified type of signal.